1. The Field of the Invention
The present invention is directed to high resolution lithographic printing plates and methods for preparing lithographic printing plates. In particular, the present invention is directed to lithographic printing plates that are suitable for imaging with laser discharge.
2. Relevant Technology
Off-set lithography is a well known method of introducing a printed image onto a recording medium using ink-accepting "oleophilic" and ink-repellent "oleophobic" surface areas. In general, off-set lithography can be performed using dry plate printing or wet plate printing. In dry plate printing, the printing plate comprises materials having oleophilic surface areas and oleophobic surface areas. Alternatively, in wet plate printing, the plate comprises materials having hydrophilic surface areas and oleophilic surface areas. An adhesive fluid (also referred to herein as "fountain solution" or "dampening solution")is applied to the wet printing plate to provide ink repellency (i.e., to make the layer oleophobic) to the hydrophilic surface area.
In off-set lithography, ink is applied to the printing plate. The ink is then drawn to the oleophilic areas of the plate and subsequently transferred to a compliant intermediate surface known as a blanket cylinder which, in turn, applies the image to a recording medium.
Off-set lithography printing plates have traditionally been produced photographically. Photographic plate making processes generally use a photographic negative to form an image on a photosensitive layer that is subjected to numerous other chemical steps (depending on the specific photographic process used, or whether a wet plate or a dry plate is formed) to form an image on the printing plate. Although lithographic printing plates can be formed using photographic plate making processes, these processes tend to be tedious, time consuming, environmentally detrimental and require facilities and equipment adequate to support the necessary chemical steps.
More recently, in an attempt to overcome the problems associated with photographic plate processes, electronic plate making processes using electromagnetic radiation pulses that can be produced by lasers have been used. Because of the ready availability of laser equipment and their amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. For example, lasers have been used to transfer thermal-transfer materials onto a suitable substrate. In the thermal-transfer process, the material transferred has a different affinity for a fountain solution and/or ink than the acceptor substrate material resulting in a printing plate having an oleophilic surface area and an oleophobic surface area (or hydrophilic and oleophilic surface areas, depending on whether a wet or dry plate is formed).
Unfortunately, due to the limited amount of material that can be effectively transferred to the acceptor substrate with acceptable adhesion using a thermal-transfer process, printing plates produced with transfer-type systems lack durability. Furthermore, because the transfer process involves melting and resolidification of the transfer material, the resolution and quality of the image produced by the printing plate is often unsatisfactory. In addition, inconsistent transfer from the donor sheet to the acceptor substrate is also often a problem.
U.S. Pat. No. 5,339,737, issued to Lewis et al. ("Lewis")provides a lithographic printing plate characterized as enabling imaging using laser-discharge. In Lewis, an image is formed on a printing plate using laser ablation technology. Referring to FIG. 1A, the printing plate 10 generally comprises a substrate 16 having a first layer 14 on the substrate that is characterized by efficient absorption of infrared ("IR")radiation. A second layer 12, described as being preferably polyvinyl alcohol, is located on first layer 14 The substrate 16 and the second layer 12 have different affinities for ink (dry-plate construction) or an adhesive fluid for ink (wet-plate construction).
Upon actuation of a properly positioned laser, laser radiation is absorbed by first layer 14, causing ablation of the first layer. Ablation of the first layer 14 disrupts the overlying second layer 12. A representation of an ablated first layer 14 and a loosened laser ejecta portion 17 of first layer 14 and second layer 12 is shown in FIG. 1B. A subsequent cleaning step is typically required to remove laser ejecta portion 17. The result is an image spot 18 (shown in FIG. 1C as being filled with ink 20) extending down to the substrate layer 16 whose affinity for the ink or ink-adhesive fluid differs from that of the second layer 12.
As used herein, the term "image spot" is defined as the image formed in the printing plate by a laser. Numerous image spots are combined to form an image or an image area. Other patents disclosing laser ablation imaging techniques to form lithographic printing plates include U.S. Pat. Nos. 5,351,617, 5,353,705 and 5,379,698.
Although the use of laser ablation techniques to form images on printing plates is a significant advancement in the imaging of printing plates, this technique still has its drawbacks. One problem with lithographic printing plates which are imaged using typical laser ablation technology, such as disclosed in Lewis, is the dramatic need for post ablative cleaning.
According to Lewis, the topmost layer often remains on the plate after the ablation process as a disrupted, but unremoved layer. In addition, ablation of the absorbing layer creates debris trapped beneath the top layer. In order to remove the debris and topmost layer, the topmost layer must be removed in an additional post-ablative cleaning step. This is disclosed as being accomplished through the use of a mechanical contact cleaning device such as a rotating brush.
Thus, there remains a need in the art for a printing plate that does not require post-ablative cleaning and wherein a high resolution image over large areas can be formed in the printing plate using laser ablation.